The invention relates to an assembly comprised of a vane and of a cooling liner for cooling the vane, in a turbomachine nozzle guide vanes assembly.
A turbomachine comprises rotor stages—compressor and/or turbine rotor stages—separated by nozzle guide vanes assemblies. The latter comprise a plurality of fixed vanes intended to guide the streams of gas. The fixed vanes extend, in the path of the gases, between an outer shroud and an inner shroud. Because of the temperature of the gases passing through them, particularly in the nozzle guide vanes assemblies separating turbine stages, the vanes are subjected to very severe operating conditions; it is therefore necessary to cool them, generally by forced convection or alternatively by the impact of air, within the vanes.
For air impact cooling it is possible to use multiperforated longitudinal liners. These liners are generally made of a heat-resistant alloy, for example one based on chromium (Cr), cobalt (Co) and nickel (Ni). A liner such as this is slid longitudinally into the cavity of a vane. It is supplied with cooling air at the outer shroud. Because of the pressure difference there is between the interior cavity of the liner and the cavity formed between the liner and the vane, some of the air is propelled, via the perforations in the liner, against the internal wall of the vane, thus cooling it. This air is then removed, along the trailing edge of the vane, by calibrated perforations, into the gas path. The remainder of the air is removed through the inner shroud to other parts of the engine that require cooling, such as the turbine disk or the bearings.
The cavity in the vane forms two openings in the inner and outer platforms. The liner is generally fixed, on the outer side, to the wall of the outer opening, by brazing or welding, for example. This then yields a kind of brazed guideway connection. The liner is also guided, at its other end portion, in the inner opening, the wall of which forms a guideway for this purpose and makes it possible to compensate for differential expansions between the liner and the vane.
According to an advantageous configuration, the liner comprises, on its outer side, a flange, brazed to the nozzle guide vanes assembly. A flange sleeve is known from document US 2002/0028133. A flanged liner displays various advantages over liners in which the outer portion is brazed to a guideway: it allows the liner to be fitted very easily in the vane, with determined radial positioning, and the brazing of the flange to the nozzle guide vanes assembly is easy to perform and can be visually checked.
It is essential to ensure a good seal between the liner and the vane at the flange. This is because were sealing not achieved satisfactorily, leaks would occur in one direction or the other each being to detrimental effect. Thus, if the pressure on the outer side of the outer shroud of the nozzle guide vanes assembly is greater than the pressure in the cavity formed between the liner and the vane, air will enter the latter cavity; this causes an increase in the pressure on the outside of the liner which means that the air has less of a tendency to be propelled from the inside of the liner against the vane and vane cooling will therefore suffer. Conversely, if the pressure in the cavity between the liner and the vane is higher than the pressure on the outer side of the outer shroud of the nozzle guide vanes assembly then the air used to cool the vane and which has therefore heated up, will escape from this latter cavity and has a detrimental effect on the cooling, afforded by other means, of the outer side of the nozzle guide vanes assembly. The above problems could be partially alleviated by increasing the volume of the cooling flow in this region but increasing the volume in one location means reducing it somewhere else.
None of these situations is satisfactory and it is necessary to have a satisfactory sealed connection at the flange.
Such a connection may be obtained by brazing. However, even though such a brazed joint can be visually checked on the flange, there is still a risk of incomplete or defective brazing, leaving the way open for possible air leaks.